1. Field of the Invention
The present invention relates to resolving conflicts when making point to multipoint calls involving first and second communication systems of equal status and which are linked by way of a virtual circuit has particular, but not exclusive, application to resolving conflicts in digital trunked private mobile radio systems.
2. Description of the Related Art
Insofar as avoiding conflicts with respect to access of both analog and digital trunked private mobile radio systems is concerned, a well known technique is dynamic framelength slotted Aloha as described and claimed for example in British Patent Specification 2 069 799B (PHN 9693). A source of conflicts which cannot be resolved by such an access protocol, however, is when two digital trunked private mobile radio systems have to communicate with each other by way of a virtual circuit across an intersystem interface and each system wants to use the virtual circuit at the same time. An example of such an arrangement is two regional police forces A and B, each having a central communication station, including a system controller and base station transceivers, and a plurality of secondary stations (mobile and portable radio units) which communicate with the central station, the respective central stations being interconnected by a virtual circuit formed by landline which constitutes the intersystem interface. Such a conflict may occur in a vehicle pursuit situation where a car from police force A crosses the regional interface into police force B's area. Whilst the pursuit was within region A communication between the pursuit vehicle, the central station and other secondary stations is controlled by the system controller at the central station in region A. Thus the system controller avoids conflicts by assigning physical channels according to predetermined procedures. However, in the event of a car from police force A entering police force B's radio coverage area, force B's central station has to provide a channel for communication between police force A's secondary station and force A's central station by means of a radio channel from force A's secondary station and force B's central station to a link between the two central stations. Additional resources may also be allocated to facilitate communication between the two central stations and with force B's secondary stations. It can occur that, within the operational parameters of each system, substantially coincident transmissions are attempted via each central station to the other central station. If the respective transmissions are of equal priority then a conflict situation arises and since there is no central arbiter, a method of resolving this conflict is desirable as an alternative to each central station instructing its secondary station to retry at a random interval later which is less efficient.